The present invention relates to a pumping-up generator/motor system for use with a pumping-up power plant, or more in particular to a pumping-up generator/motor system using a wound-rotor type induction machine.
Prior to explanation of a conventional method of operating a generator/motor system of this type such as a supersynchronous Scherbius system, explanation will be made about a system therefor. Such a system as disclosed in U.S. Pat. No. 4,481,455 issued to Sugimoto et al. on Nov. 6, 1984 includes a generator/motor 2 selectively driving a water turbine 1 as a load machine in a motor mode or driven by a prime mover in a generator mode as shown in FIG. 6. This generator/motor has a structure of a wound-rotor type induction machine with and its rotor is connected to cyclo-converters 3 through slip rings 4. Each cyclo-converter 3 is connected to a power line system through a transformer 5.
In operation thereof, each cyclo-converter is controlled, when the generator/motor is driven in a motor mode, so as to control the rotational speed of the motor and, when driven in a generator mode, so as to regulate the frequency of the generator output at a constant level regardless of the rotational speed of the generator.
In this configuration and operation, however, when the cyclo-converter is driven at a vicinity of its synchronous speed (where the slip is small), the polarity or direction of the current flowing in the cyclo-converter is changed at a relatively long time interval and hence the current of the cyclo-converter flows mainly through one side of the inverse-parallel connected rectifiers of the cyclo-converter. In order to prevent the cyclo-converter from being overheated, therefore, it is required to reduce the output of the generator/motor when it is driven at a vicinity of the synchronous speed where slip is small, or if it is desired not to reduce the output of the generator/motor, it is necessary to use a cyclo-converter of a large capacity.
In the pumping-up power plant, on the other hand, it is common practice to set the rated water flow rate in operation of the generator mode almost to the same value as that in operation of the motor mode, that is, pumping-up mode. In the case of a turbo machine typically such as a pump-turbine, the rotational speed N.sub.G, at which the rated output in the power generation mode is produced at the maximum efficiency, is generally lower than the rotational speed N.sub.P at which the rated output in the pumping-up mode is produced. As a result, it is necessary that the speed range of the variable-speed type pumping-up generation system covers the rotational speeds N.sub.G and N.sub.P.
Specifically, if the maximum efficiency of the water turbine is to be maintained in variation of its water head or output in the generator mode, it is necessary to maintain the speed variation in a vicinity of the rotational speed N.sub.G. When the pumping-up output is controlled in the pumping-up mode for improving the stability of the power system, on the other hand, it is necessary to maintain the speed variation in a vicinity of the rotational speed N.sub.P. Generally, the value N.sub.G or N.sub.P is set to a level approximate to the maximum speed in the control range of the water turbine in the generator mode pumping-up mode, respectively.
Now, the relationship between the speed control range and the rotational speed N.sub.G or N.sub.P will be explained more in detail with reference to FIGS. 2 and 3. In the case of a supersynchronous Scherbius system of variable-speed generator/motor, the required capacity of the converter units, such as thyristors, in inverse-parallel connection of a cyclo-converter is proportional to the maximum value Smax of the slip S=(N.sub.O -N/N.sub.O), where N is the rotating speed and No is the synchronous speed. If the required capacity of the thyristor elements is to be minimized, therefore, it is necessary to set the synchronous speed N.sub.o at a value between the maximum speed Nmax and the minimum speed Nmin in the desired speed range of the generator/motor as shown in FIG. 3 so that the maximum slip Smax becomes smallest with a speed range covering the maximum and minimum speeds Nmax and Nmin.
When a cyclo-converter is used and the generator/motor is driven in a vicinity of the synchronous speed, however, the output frequency of the cyclo-converter is very small near to a direct current, resulting in the necessity of either reducing the output of the generator/motor or increasing the capacity of the cyclo-converter as mentioned above.
For this reason, in another conventional method, the speed range is set below the synchronous speed in the generator mode as shown in FIG. 4. In this method, however, as compared with the arrangement of FIG. 2 assuming that the maximum slip set value Smax and the capacity of the converter elements are made the same, the speed control range in the generator mode is narrowed thereby to reduce the average efficiency.
A generator/motor of this type using a cyclo-converter is disclosed in "IEEE Transactions on Power Apparatus and Systems", Vol. PAS-100, No. 5, May 1981 (Pages 2171 to 2176) and Vol. PAS-99 No. 5, Sept/Oct 1980 (Pages 1828 to 1837).